Bushing



Jan. 5, 1937. F, J, VOGEL l 2,066,917

BUSHING Filed May 1l, 1934 i, WITNESSES: NVENTOR 7 /j BY 7"/ uq/(L. VMM/pil) AT NEY Patented J an. 5v, 1937 Busi-mrc.

Fred J. Vogel, Sharon, Pa., assigner to West-l inghouse Electric & Manufacturing Company,

East Pittsburgh, Pa., Vania Application May 11,

8 Claims.

10 ings so as to iiashover, rather than fail through the insulation, when subjected eithe'rto abnormally high voltages of commercial frequency or to surges of steep wave front, such as may be produced as a result of switching operations orv lightning, for example.

To insure the necessary impulse characteristics of the bushing, astandardized -surge test is applied which comparesthe surge characteristics of the bushing, for a standardized vsurge wave, with the corresponding characteristic of a standard point gap, different lengths of 'gap being used for different bushing voltage'ratings. Wet and dry ashover tests -at commercial frequency are also applied to insure the necessary bushing strength at low frequencies.

In the bushing, the strength of the insulation must be higher than that of the air gap between the cap and ange, under both extreme conditions of impulse voltages, and voltages of commercial frequency. Also, the strength of the air gap between cap and flange must exceed test standards under both extreme conditions, in order to insure proper operation of the bushing. It has not heretofore been practicable to independently control the flashover values of the air gap, and it has been necessary, therefore, to provide suicient insulation to withstand the particular test condition which is most severe for a given bushing design. For example, if the negative impulse ashover value is high in a given design, it is necessary to provide suilicient insulation to insure flashover under this particular test condition. The present bushing designs usually have large diameter, well-rounded cap and flange surfaces and suicient insulation for strength against impulse or surge voltages.

I have found, however, that by modifying the shape of the cap and flange it is possible to obtain impulse characteristics similar to those ofl a point gap, and at the same time preserve the flashover strength at low frequencies which is inherent in a rounded surface design. In this way an advantage in design is secured which may be realized in operating a given bushing at 55 higher voltage, in reducing the amount of ina corporation of Pennsyl- 1934, serial No. 1225,09;

(c1. 11s-31s) sulation in the bushing or in minimizing the insulation strain onv the bushing.

In accordance with the principle `of my ln- 'vention, a protuberance orprojection is provided on the cap for producing a point of maximum voltage gradient. The parts are arranged so that this point of maximum voltage gradient lies in a line of electric force whichterminates in an area of low .voltage gradient of the ange, such as flat or rounded surface of the latter. A protuberance o'rv projectionpn the flange similarly lies'in aline of velectric force which terminates in a flat r'roundedsurface of the cap. With this arrangement, positive ilashovers from the cap to flange are initiated from the projection on the cap, and negative flashovers are initiated ,from the projection on vfthe ange and follow a' diiferent path. The gaps formed in this way have a pointed electrode at the terminal where the gap initially breaks down and a fiat or rounded electrode at the other terminal. They withstand impulse voltages with a time characteristic similar .to that yof a point gap, and permit of independent adjustment of positive and negative impulse ilashover values. At the same time, .the presence of the projections does not greatly alter wet and dry ilashover values at commercial frequency. l

It is accordingly an object of my invention to provide a novel method and means for controlling the ilashover characteristics of insulating devices such as bushings.

Another object of my .invention is to providey a novel condenser-type bushing having desirable flashover characteristics.

Other objects of my inventionwill become evident from the following detailed description taken in conjunction with the accompanying drawing, in which Figure l is an elevational view of a condensertype bushing embodying my invention, with partsbroken away.

Fig. 2 is a fragmentary view showing diagrammatically the distribution of voltage in the air gaps of the bushing shown in Fig. l.

Figs. 3 and 4 are fragmentary diagrammatic views showing modifications of the bushing shown in Fig. 1.

Referring to Fig. 1, the main conductor or stud I, which may be a cylindrical bar or tube of copper, is surrounded by a built up condenser structure 2 consisting of alternate cylindrical layers of dielectric material and metal foil, constructed in a manner well understood in the art. The condenser structure 2 is secured Within a metal flange 3 by any suitable means (not shown). The fiange 3 is provided with a machined upper surface upon which a gasket 4 is mounted as a base for a porcelain weather casing 5. A second gasket 5 is provided on the top of the weather casing 5, to support the cap structure 1.. An electrode 8, having an upwardly projecting sharp or square-edged portion 9, is secured to the flange 3.

The cap structure I comprises an expansion chamber I0, of metal, having a rounded outer surface, and an expansion chamber nut I I for holding the parts tightly together. Various packing elements, known in the art, for providing a fiuid-tight seal between the expansion chamber I0 and the stud I, have .for simplicity been omitted from the drawing. An electrode I 2 having a depending square or Sharp edged portion I3, similar to the edge- 9, is mounted on the stud I and secured thereto by a nut I4.

'I'he space between the condenser structure 2 and the weather casing'5 is filled with a suitable insulating compound, as shown at I5.

The condenser structure 2, in the bushing shown in Fig. 1,'is of the uniform layer" type. In a uniform layer condenser-type bushing, the layers are of equal capacitance, and successive layers differ in length by a fixed increment. The distribution of the condenser layers has a considerable eecty upon the'voltage gradients in the electrostatic field ofthe bushing. The manner `in Ywhich the invention maybe practiced with condenser-type bushings having nonuniform layers will be explained in connection with Figs. 3 and 4.

Referring to Fig. 2, which shows diagrammatioally the portions of the electrostatic eld in the air gaps associated with the electrodes 8 and I2, equipotential surfaces are indicated by solid curve fragments I 6, and lines of electrostatic force by dotted lines H+ and I'I. It will be understood that a line of force -is a line which is at any point perpendicular to the equipotential surface of the electric field which passes through the point.

In accordance with my invention, the sharp edge I3 of the electrode I2 produces the maximum potential gradient at the surface of the cap` structure 1. Similarly, the sharp edge 9 produces the maximum potential gradient at the surface of the flange 3. The parts are arranged, however, so that the lines of force which pass through the points of maximum potential gradient terminate on surfaces of low potential gradient at the other end of the electric eld. In the arrangement shown, the two gaps are on opposite sides of the bushing, but obviously the result desired can be attained with other arrangements.

The edges 9 and I3 are preferably located at such positions in the electric field that their maximum potential gradients are equal and of a value corresponding to the desired flash-over strength of the bushing for impulse voltages. For example, if it is desired to have the bushing flash over at a voltage 15% above the breakdown voltage of a standard point gap, the edges 9 and I3 are located so that the same breakdown potential gradient as in the point gap is secured at a voltage 15% above the point gap characteristic.

'Ihe operation of the bushing is as follows: Positive overvoltage conditions of the cap structure 'I cause the left hand gap to flash over. Similarly, negative over-voltages of the cap structure 1, cause the right hand gap to iiash over.

When the invention is applied to non-uniform layer condenser-type bushings, the edges I3 and 9 are located at different positions with reference to the condenser layers, to attain the desired conditions of voltage gradient. Arrangements ofthe upper electrode I2 for two forms of non-uniform layer bushing are indicated in Figs. 3 and 4.

I do not intend that the present invention shall be restricted to the specific structural details or arrangement of parts herein set forth, as various modifications thereof may be effected without departing from the spirit and scope of my invention. I desire, therefore, that only such limitations shall be imposed as are indicated in the appended claims.

I claim as my invention:

l. In an insulating device, two conducting elements shaped to provide non-uniform potential gradients over the surfaces thereof, and insulating means separating said elements, said elements being arranged so that the maximum gradient point of each of said surfaces lies in a line of electrostatic force which terminates at a region of low potential gradient at the other of said surfaces.

2. In an insulating device, a rst Velement having a projection to provide a point of maximum potential gradient, a second element shaped to provide a surface of4 low potential gradient, said first element being subject to positive voltages with reference to said second element, insulating means separating said elements, saidelements being arranged so that said projection lies in a line of electrostatic force 'which terminates in said surface, and means for preventing a positive flash-over from said second element to said projection.

3. In an insulating device, a first element having a projection to provide a point of maximum potential gradient and having a surface of low potential gradient, a second element having a second projection to provide a point of maximum potental gradient and having a second surface of lowpotential gradient, and insulating means separating said elements, said elements being arranged so that said first-mentioned projectionand said second surface lie in one discharge path, and said second projection and said firstmentioned surface lie in another discharge path.

4. In an insulating bushing, a cap structure having a projection thereon to provide a point of maximum potential gradient, a flange structure having a surface region of low potential gradient, an insulating structure separating said cap and flange structures, said structures being arranged so that said point of maximum potential gradient lies in a line of electrostatic force which terminates in said region of low potential gradient, and means for preventing a positive flash-over from said surface region to' said projection.

5. In an insulating bushing, a cap structure having a surface region of low potential gradient, a fiange structure having a projection thereon to provide a point of maximum potential gradient, an insulating structure separating said cap and ange structures, said structures being arranged so that said point of maximum potential gradient lies in a line of lelectrostatic force which terminates on said region of low potential gradient, and means for preventing a positive flash-over from said surface region to said projection.

6. In an insulating device, a ilrst conducting member, a second conducting member, insulating means separating said members, means for producing a point of maximum potential gradient adjacent the surface of said first member, said point of maximum potential gradient lying in a line of electrostatic force which terminates in a region of low potential gradient on said second member, and means for producing a second point of maximum potential gradient adjacent said second member, said second point of maximum potential gradient lying in a line of electrostatic force which terminates in a region of low potential gradient on said rst member.

7. In an insulating bushing, a capstructure and a flange structure each having a sharpedged projection to provide points of maximum potential gradient, and an insulating structure separating said cap and ilange structures, said structures being arranged so that said projections lie in diierent discharge paths between said cap and flange, and, for a given bushing voltage, the potential gradient at each of said projections is less than the corresponding potential gradient of a pre-selected testing point gap for the same voltage, whereby the positive and negative impulse ashover characteristics of said bushing are each similar to and higher than the corresponding characteristics of said point gap and may be independently controlled.

8. In an insulating bushing of the condenser type, a cap structure and a ange structure each having a sharp-edged projection to provide points of maximum potential gradient, and an insulating condenser structure separating said cap and flange structures, said structures being arranged so that said projections lie in different discharge paths between said cap and flange, and, for a given bushing voltage, the potential gradient at each of said projections is less than the corresponding potential gradient oi' a, preselected testing point gap for the same voltage, whereby the positive and negative impulse ashover characteristics of said bushing are each similar to and higher than the corresponding characteristics of said point gap and may be independently controlled.

FRED J. VOGEL.

DISCLAIMER 2,066,917.Fred J. Vogel, Sharon, Pa.

di: Manufacturing Company.

BUsHING. Disclaimer filed January 14, 1938, by the Patent dated January 5, 1937. assignee, Westinghouse Electric Hereby enters this disclaimer to claims 1, 2, 3, and 6 in said specification.

[Oficial Gazette February 22, 1938.] 

